1. Technical Field
This invention relates to the technology of treating exhaust from a compressed natural gas (CNG) fueled engine to remove its noxious content, and more particularly to the treatment of exhaust from a CNG engine controlled to operate under lean-burn combustion conditions.
In copending U.S. Ser. No. 07/789,707, filed Nov. 8, 1991, authored by some of the authors of this invention and commonly assigned to the assignee herein, a catalyst is disclosed which enhances the three-way conversion capability of a modified Pd/Al.sub.2 O.sub.3 catalyst in treating the exhaust gas of a compressed natural gas-fueled engine, provided the engine is limited to being operated slightly rich of stoichiometry, i.e., redox ratio (R) of 1-1.2 (R being the ratio of reducing components to oxidizing components in the exhaust gas). Untreated exhaust from a CNG-fueled engine, operated under rich conditions, contains a high content of CO (about 2000-2250 ppm), a high content of NO.sub.x (at least about 450 ppm), and a methane content at least about 300 ppm. Although the enhancement achieved by this disclosure over the prior art is significant, fuel-rich operation affects the fuel economy of the CNG-fueled engine and therefore can be undesirable. At stoichiometry or below stoichiometry (i.e., lean region), the conversion capability of such a catalyst drops dramatically.
If the exhaust gas is pretreated by use of a copper-exchanged zeolite, prior to entering the three-way/CNG catalyst described above, the engine can be operated at stoichiometry to achieve conversion efficiency in excess of 80% for all of CO, NO, and CH.sub.4 (see copending U.S. Ser. No. 07/789,558, now U.S. Pat. No. 5,197,053 authored by some of the authors of this invention, and commonly assigned to the assignee herein). However, if this combination is used to treat the exhaust from a fuel-lean operated CNG engine, the conversion capability drops again dramatically. Moreover, expensive electronic controls are required to regulate the engine operation at stoichiometry.
2. Discussion of the Prior Art
Copper-exchanged zeolites have been used to cleanse lean-burn type exhaust, but only from exhaust gases simulating the exhaust from a conventional gasoline-fueled engine. Such gasoline engine exhaust contains very high contents of fast-burning hydrocarbons, a representative of which is propylene (at about 1000 ppm), high contents of slow-burning hydrocarbons, a representative of which is propane (at about 500 ppm), very high contents of NO (about 1000 ppm), and very high content of CO (at about 15,000 ppm), with an absence of methane. A copper-exchanged zeolite catalyst would not be effective, by itself, in treating the total exhaust from a CNG-fueled engine operating under lean conditions, since such an exhaust would contain considerably lower amounts of NO and CO but significant amounts of methane. The conversion efficiency would be well below 80% (see Li et al, "Stoichiometric Catalytic Decomposition of Nitric Oxide Over Cu-ZSM-5 Catalyst", Journal of Physical Chemistry, Vol. 94, p. 6145, 1990; Iwamoto et al, "Influence of SO.sub.2 On Catalytic Removal of NO Over Copper Ion-Exchange ZSM-5 Zeolite", Applied Catalysis, Vol. 69, L 15-L 19, 1991; and Hamada et al, "Highly Selective Reduction of Nitrogen Oxides With Hydrocarbons Over H-Form Zeolite Catalysts In Oxygen-Rich Atmospheres", Applied Catalysis, Vol. 64, L 1-L 4, 1990).
What is needed is a catalyst system that economically and durably converts CO, NO.sub.x, and CH.sub.4 present in the exhaust of a lean-burn CNG-fueled engine.